1. Field of the Invention
This invention relates to fine wire bonders employed to make electrical connections between electrodes or pads on semiconductor devices and lead out pads. More specifically the present invention relates to an apparatus for detecting whether the bonder has made a proper bond on the electrodes or pads.
2. Description of the Prior Art
One of the problems associated with making fine wire bonds on semiconductors is the desirability of determining whether a good or proper bond is made. Bad bonds may be caused by improper deformation of the wire being bonded, improper adhesion of the wire to the pads or electrodes of the semiconductor device being bonded, or no bond may occur because the wire is missing. Numerous solutions have been proposed for automatically determining whether a proper bond has been made. Since the wire being bonded at first bond is looped and again bonded at the second bond, a length of wire is employed to make the loop. It has been suggested that the wire spool on which the bond wire is stored must move in order to supply the length of wire being employed to make the interconnection between the first and second bond, thus, sensing the spool motion is one way of determining whether the first and second bond was made. However, this does not give any indication of the quality of the bond and most probes and indicators formerly employed created inertia and drag on the wire spool which created as many problems as solutions.
In order to overcome the problems associated with sensing the motion of the spool on which the fine wire is stored, attempts were made to positively drive and pay out fine wire from the spool employing a drive motor on the spool. This method of attempting to detect missing wires does not give any indication of the quality of the bond. This method requires placing tension in the wire path between the spool and the bonding wedge. It is possible to get a kick-back or undesired pull on the wire which will remove the fine wire from the bonding wedge and cause a missing wire.
It has been suggested that the resistance at the interface of the bond and the electrode on a semiconductor may be monitored so as to determine whether the bond has been properly made. However, employing electrical energy in the wire at the wire bond may possibly damage the semiconductor device.
It is known that the current being supplied to the ultrasonic transducer which is employed to make ultrasonic wire bonds has a distinctive pattern which may be employed to determine whether a good wire bond has been made. For example, the current supplied the ultrasonic transducer increases up to a time which is approximately half of the bonding time and if a good bond is being made the curve slowly falls off requiring less current drive to the transducer. If the bond is being made without wire the current being supplied the ultrasonic transducer continues to increase out to the end of the bonding time. This principle has been employed to detect whether proper bonds are being made. However, when a curve of current versus time is employed to detect whether a wire bond is being made properly, it requires that a graphic plot be made and high and low points on the current versus bonding time curve be designated in order to establish logic levels for sensing whether a good bond is made. This method of graphically setting high and low logic points on a current versus bonding time curve has other problems because the curves may have reverse inflection points and differ from bond to bond.
It would be desirable to provide an apparatus for automatically determining whether a good fine wire bond is being made by an ultrasonic wire bonder during the time that the bond is actually being made. It would also be extremely desirable to provide a simplified structure for indicating the quality of each bond being made while automatically determining whether the wire is missing under the working face of the bonding tool.